The evolution characteristics and stability analysis of Jinshan Trough in the north shore of Hangzhou Bay during 1972 to 2013

The erosion-accretion evolution pattern and mechanism within the Jinshan Trough is analyzed using GIS technology,aiming to imply the sediment transport characteristics in a complex hydrodynamic condition from a spatial-temporal view. Data during 1972-2013 was collected. Analysis results show that: (1) In the long term,the evolution in areas deeper than 15 m is dominated by erosion,accounting for 88.05% of the total trough area,and the erosion amount (120.35×10⁶ m³) is larger than that of accretion (3.76×10⁶ m³),with the average net erosion rate reaching up to 7.46 cm/a. Generally,for the Jinshan Trough as a whole,it is extending lengthwise and broadening southward obviously. The maximum length of it increased by 7.34 km; the maximum width has also nearly 1 km. Nevertheless,the anti-scouring deposit layer in the Jinshan Trough has somehow promoted a rough equilibrium of erosion and accretion even with continuing effects of the strong tidal currents during the last four decades. Besides,the average depth ranges from 21.0 to 23.59 m with a slight fluctuating amplitude,and the maximum depth from 46.90 to 51.00 m and are spatially aggregated. (2) With a basis of specific topography,the short-term erosion-accretion pattern are drastically affected by the water and sediment condition changes due to human activities. During 2002 to 2012,the net erosion rate is as high as 22.14 cm/a. On the whole,the erosion and accretion pattern in the Jinshan Trough keeps balanced during 2012 to 2013. (3) By analyzing the swinging values of the central axis in areas deeper than 15 m,20 m and 25 m respectively,it is revealed that most parts of the Jinshan Trough are in a stable state,which accounts for more than 65% of the whole trough. However,unstable areas are distributed at the west of the Sixth Coastal Levee and the city beach,where the water depth fluctuates with a larger amplitude. As a proposal provided by our research,intent hydrographic monitoring and accurate evaluation of seawalls' safety are required for the near future.